Matlab IIR滤波器传递函数

Question

I am using MATLAB to design a filter and obtain the transfer function coefficients a and b in order to then code the filter in C.

However I have noted that the function sos2tf is creating different values than expected. In fact when using fvtool to view the filter using the b and a coefficients the filter looks different than that of I plot it using the sos matrix or the actual filter f .

If you try the below code you can understand better what I am saying. Is this a function problem coming from MATLAB? Can I use substitutes to sos2tf which gives me the required filter coefficients? Or can I retrieve them directly without first using sos ?

Note that I have already tried using designfilt and then using the tf function and the resulting coefficients came to be exactly equal to those obtained with the code below. Thanks for your help

H = fdesign.lowpass('Fp,Fst,Ap,Ast',60,75,1,60,10000);
f = design(H,'butter','MatchExactly','passband');  % Design an FIR BW filt
info(f)                   % View information about filter
fvtool(f);
[b,a] =  sos2tf(f.sosMatrix);
fvtool(b,a);

Answer 1

I don't know exactly what the problem is, as it isn't working for me either, but I'm going to give you a workaround to get the filter coefficients. In Matlab Command Window type fdatool,a filter design app window will pop up, design your filter there by properly choosing the desired parameters. Once you're done, in that fdatool window go to File->Export->Export As-> Coefficients. From that very same tool you can select File-> generate matlab code-> Filter design function (or any of the 3 options). And that way you eliminate possible bugs there might be in the code you posted in your question. That's how I do it and it has always worked well